Detection of exhaust particulate filter substrate failure

ABSTRACT

A method of detecting failure of a substrate in a particulate filter includes comparing an absolute value of a difference between a theoretical temperature difference and an actual temperature difference between an upstream end and a downstream end of the particulate filter to a temperature differential threshold, and comparing an absolute value of a difference between a theoretical pressure difference and an actual pressure difference between the upstream end and the downstream end of the particulate filter to a pressure differential threshold. Failure of the substrate is indicated by one or both of the temperature difference and the pressure difference being greater than the temperature differential threshold and the pressure differential threshold respectively.

TECHNICAL FIELD

The invention generally relates to a method of detecting failure of a substrate in a particulate filter of an exhaust system of a vehicle.

BACKGROUND

An exhaust system for a vehicle may include a particulate filter. If the engine includes a diesel engine, then the particulate filter is commonly referred to as a diesel particulate filter. The particulate filter filters particulate matter, i.e., soot, from the exhaust gas of the engine. The particulate filter may include one or more substrates that define a plurality of apertures, through which the exhaust gas must flow. The particulate matter collects on the substrate as the exhaust gas flows through the apertures. The particulate filter is occasionally regenerated to remove the collected particulate matter. Regeneration of the particulate filter includes heating the particulate filter to a temperature sufficient to burn the collected particulate matter, which converts the particulate matter to carbon dioxide that dissipates into the atmosphere.

An on board diagnostic system may monitor the status of the particulate filter to determine if the particulate filter, and specifically the substrate, has failed. Failure of the substrate may include, but is not limited to, damage to the substrate or removal of the substrate.

SUMMARY

A method of detecting a failure in a substrate of a particulate filter in an exhaust system of a vehicle is provided. The method includes calculating an absolute value of a difference between a theoretical temperature difference between an upstream end and a downstream end of the particulate filter, and an actual temperature difference between the upstream end and the downstream end of the particulate filter, and comparing the absolute value of the difference between the theoretical temperature difference and the actual temperature difference to a temperature differential threshold to determine if the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is greater than the temperature differential threshold. The method further includes calculating an absolute value of a difference between a theoretical pressure difference between the upstream end and the downstream end of the particulate filter and an actual pressure difference between the upstream end and the downstream end of the particulate filter, and comparing the absolute value of the difference between the theoretical pressure difference and the actual pressure difference to a pressure differential threshold to determine if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is greater than the pressure differential threshold. The method further includes indicating a substrate failure if the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is greater than the temperature differential threshold, or if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is greater than the pressure differential threshold.

A method of detecting a failure in a substrate of a particulate filter in an exhaust system of a vehicle is also provided. The method includes measuring the actual temperature difference between an upstream end and a downstream end of the particulate filter, calculating an absolute value of a difference between a theoretical temperature difference between the upstream end and the downstream end of the particulate filter and the actual temperature difference between the upstream end and the downstream end of the particulate filter, and comparing the absolute value of the difference between the theoretical temperature difference and the actual temperature difference to a temperature differential threshold to determine if the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is greater than the temperature differential threshold. The method further includes comparing a measured particulate matter level of particulate matter trapped in the particulate filter to a particulate matter threshold to determine if the particulate matter level is greater than the particulate matter threshold. The method further includes measuring the actual pressure difference between the upstream end and the downstream end of the particulate filter, calculating an absolute value of a difference between a theoretical pressure difference between the upstream end and the downstream end of the particulate filter and an actual pressure difference between the upstream end and the downstream end of the particulate filter, and comparing the absolute value of the difference between the theoretical pressure difference and the actual pressure difference to a pressure differential threshold to determine if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is greater than the pressure differential threshold when the particulate matter level is less than the particulate matter threshold. The method further includes indicating a substrate failure if the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is greater than the temperature differential threshold, or if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is greater than the pressure differential threshold.

Accordingly, both the temperature differential and the pressure differential between the upstream end and the downstream end of the particulate filter are examined to detect a failure of the substrate in the particulate filter. A thermal mass of the particulate filter causes the temperature downstream of the particulate filter to be less than a temperature upstream of the particulate filter. During normal operation of the particulate filter, the temperature differential between the upstream end and the downstream end of the particulate filter falls below a temperature differential threshold. Damage too or removal of the substrate causes the temperature differential between the upstream end and the downstream end of the particulate filter to rise above the temperature differential threshold. Similarly, a flow restriction in the particulate filter causes a pressure downstream of the particulate filter to be less than a pressure upstream of the particulate filter. During normal operation of the particulate filter, the pressure differential between the upstream end and the downstream end of the particulate filter falls below a pressure differential threshold. Damage to or removal of the substrate causes the pressure differential between the upstream end and the downstream end of the particulate filter to rise above the pressure differential threshold. Accordingly, comparing the actual temperature differential and the actual pressure differential the temperature differential threshold and the pressure differential threshold respectively may indicate damage too or removal of the substrate. Using both the temperature and the pressure of the particulate increases the ability to detect damage and/or removal of the substrate.

The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an engine and an exhaust system of a vehicle.

FIG. 2 is a flow chart showing a method of detecting failure in a substrate of a particulate filter of the exhaust system.

DETAILED DESCRIPTION

Referring to FIG. 1, wherein like numerals indicate like parts throughout the several views, an exhaust system is generally shown at 20. The exhaust system 20 is coupled to an engine 22 of a vehicle. The engine 22 may include, but is not limited to, a diesel engine 22. Fuel ignites within a plurality of cylinders (not shown) of the engine 22, producing a flow of exhaust gas that is directed through the exhaust system 20 in a direction indicated by arrow 23. The exhaust system 20 treats the exhaust gas to reduce undesirable emissions, and remove particulate matter, i.e., soot, from the exhaust gas.

The exhaust system 20 may include an oxidation catalyst 24. The oxidation catalyst 24 includes a flow-through honeycomb structure that is covered with a chemical catalyst. The chemical catalyst may include a precious metal, including but not limited to, platinum or palladium. The chemical catalyst, when heated to a light-off temperature, interacts with and oxidizes pollutants in the exhaust gas, such as carbon monoxide and unburned hydrocarbons, thereby reducing undesirable emissions. The oxidation catalyst 24 may include any suitable type of oxidation catalyst 24, and may be sized and or configured in any suitable manner required to meet specific design parameters.

The exhaust system 20 may further include a Selective Catalytic Reduction (SCR) system 26. The SCR system 26 includes an exhaust fluid injector 28, which injects an exhaust fluid, such as but not limited to a mixture of urea and water, into the flow of exhaust gas. A mixer 30 mixes the exhaust fluid with the exhaust gas. When heated by the exhaust gas, the exhaust fluid forms ammonia. The SCR system 26 further includes a converter 32. The converter 32 includes a catalyst that causes or accelerates a chemical reaction between the ammonia created by the exhaust fluid and the NOx (nitrogen oxides) in the exhaust gas to form nitrogen and water vapor.

The exhaust system 20 further includes a particulate filter 34. The particulate filter 34 filters particulate matter, i.e., soot, from the exhaust gas of the engine 22. The particulate filter 34 may include one or more substrate 36 that define a plurality of apertures, through which the exhaust gas must flow. The particulate matter collects on the substrate 36 as the exhaust gas flows through the apertures. The particulate filter 34 is occasionally regenerated to remove the collected particulate matter. Regeneration of the particulate filter 34 includes heating the particulate filter 34 to a temperature sufficient to burn the collected particulate matter to carbon dioxide.

As shown, the particulate filter 34 is disposed downstream of the converter 32. The particulate filter 34 includes an upstream end 38, disposed between the converter 32 and the particulate filter 34, and a downstream end 40, disposed opposite the upstream end 38 of the particulate filter 34.

The exhaust system 20 further includes a first temperature sensor 42 and a second temperature sensor 44. The first temperature sensor 42 is disposed adjacent the upstream end 38 of the particulate filter 34, and the second temperature sensor 44 is disposed adjacent the downstream end 40 of the particulate filter 34. The first temperature sensor 42 measures a temperature of the exhaust gas upstream of the particulate filter 34. The second temperature sensor 44 measures a temperature of the exhaust gas downstream of the particulate filter 34. The first temperature sensor 42 and the second temperature sensor 44 may include any suitable temperature sensor capable of sensing the temperature of the exhaust gas within the exhaust system 20. Because the particulate filter 34 includes a thermal mass, the particulate filter 34 absorbs heat from the exhaust gas as the exhaust gas flows through the particulate filter 34. Accordingly, during normal operation, the temperature of the exhaust gas at the second temperature sensor 44 is less than the temperature of the exhaust gas at the first temperature sensor 42. When the particulate filter 34, and particularly the substrate 36 of the particulate filter 34, is operating properly, a temperature differential between the first temperature sensor 42 and the second temperature sensor 44 falls within a given temperature range for given operating conditions. Accordingly, if the temperature differential is outside the given temperature range for a specific operating condition, and particularly greater than an upper temperature threshold, then it is likely that the thermal mass of the particulate filter 34 has been altered. For example, removal and/or damage of the substrate 36 may cause the temperature differential between the first temperature sensor 42 and the second temperature to fall outside the given temperature range.

The exhaust system 20 further includes a first pressure sensor 46 and a second pressure sensor 48. The first pressure sensor 46 is disposed adjacent the upstream end 38 of the particulate filter 34, and the second pressure sensor 48 is disposed adjacent the downstream end 40 of the particulate filter 34 The first pressure sensor 46 measures a fluid pressure, i.e., gas pressure, of the exhaust gas upstream of the particulate filter 34. The second pressure sensor 48 measures a fluid pressure, i.e., gas pressure, of the exhaust gas downstream of the particulate filter 34. The first pressure sensor 46 and the second pressure sensor 48 may include any suitable pressure sensor capable of sensing the fluid pressure of the exhaust gas within the exhaust system 20.

Because the particulate filter 34 redirects the flow of exhaust gas through the apertures of the substrate 36, the particulate filter 34 decreases the fluid pressure, i.e., the gas pressure, as the exhaust gas flows through the particulate filter 34. Accordingly, during normal operation, the pressure of the exhaust gas at the second pressure sensor 48 is less than the pressure of the exhaust gas at the first pressure sensor 46. When the particulate filter 34, and particularly the substrate 36 of the particulate filter 34, is operating properly, a pressure differential between the first pressure sensor 46 and the second pressure sensor 48 falls within a given pressure range for given operating conditions. Accordingly, if the pressure differential is outside the given pressure range for a specific operating condition, and particularly greater than an upper pressure threshold, then it is likely that the flow path of the exhaust gas through the particulate filter 34 has been altered. For example, removal and/or damage of the substrate 36 may cause the pressure differential between the first pressure sensor 46 and the second pressure sensor 48 to fall outside the given pressure range.

Referring to FIG. 2, a method of detecting a failure in the substrate 36 of the particulate filter 34 in the exhaust system 20 of the vehicle is shown generally at 50. The method includes defining a theoretical temperature difference between the upstream end 38 and the downstream end 40 of the particulate filter 34, block 52. The theoretical temperature difference is the temperature difference between the first temperature sensor 42 and the second temperature sensor 44 that should occur if the particulate filter 34 and particularly the substrate 36 are operating properly.

Defining the theoretical temperature difference may include calculating a theoretical upstream temperature and a theoretical downstream temperature of the exhaust gas for the current operating conditions of the exhaust system 20. The theoretical downstream temperature is subtracted from the theoretical upstream temperature to obtain the theoretical temperature difference. For example, an equation may be generated to calculate the theoretical upstream temperature and the theoretical downstream temperature based on the mass of the exhaust gas, the flow rate of the exhaust gas, temperature of the exhaust gas upstream of the particulate filter 34, the time over which the theoretical temperature difference is calculated, or some other known variable of the exhaust system 20.

Alternatively, defining the theoretical temperature difference may include referencing stored temperature values from a table of temperature values for the current operating conditions of the exhaust system 20 to determine the theoretical upstream temperature and the theoretical downstream temperature of the exhaust gas. The theoretical downstream temperature is subtracted from the theoretical upstream temperature to obtain the theoretical temperature difference. The theoretical upstream temperature and the theoretical downstream temperature for specific operating conditions may be determined, for example, through testing at various operating conditions. These values may be stored in memory of an engine control unit, and may be referenced to determine the theoretical upstream temperature and the theoretical downstream temperature of the exhaust gas for the current operating conditions of the engine 22 and the exhaust system 20. Accordingly, the engine control unit may learn the current operating conditions of the engine 22 and the exhaust system 20, and use the current operating conditions to reference the temperature table to determine the theoretical upstream temperature and the theoretical downstream temperature of the exhaust gas.

The method further includes measuring the actual temperature difference between the upstream end 38 and the downstream end 40 of the particulate filter 34, block 54. The first temperature sensor 42 measures an actual upstream temperature of the exhaust gas, and the second temperature sensor measures an actual downstream temperature of the exhaust gas. The actual downstream temperature is subtracted from the actual upstream temperature to obtain the actual temperature difference between the upstream end 38 and the downstream end 40 of the particulate filter 34.

The method further includes calculating an absolute value of the difference between the theoretical temperature difference between the upstream end 38 and the downstream end 40 of the particulate filter 34, and the actual temperature difference between the upstream end 38 and the downstream end 40 of the particulate filter 34, block 56. The absolute value is the real number value of the difference, regardless of a positive or negative sign. Accordingly, the actual temperature difference is subtracted from the theoretical temperature difference to obtain the temperature difference between the theoretical temperature difference and the actual temperature difference. The absolute value is then taken of the temperature difference between the theoretical temperature difference and the actual temperature difference.

The method further includes defining a temperature differential threshold, block 58. The temperature differential threshold is an upper temperature limit associated with proper operation of the particulate filter 34 and/or the substrate 36. Accordingly, a temperature below the temperature differential threshold is indicative of proper functioning of the particulate filter 34 and/or the substrate 36, whereas a temperature above the temperature differential threshold is indicative of a failure of the particulate filter 34 and/or the substrate 36.

The method further includes comparing the absolute value of the difference between the theoretical temperature difference and the actual temperature difference to the temperature differential threshold, block 60. The absolute value of the temperature difference is compared to the temperature differential threshold to determine if the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is greater than the temperature differential threshold.

If the absolute value of the temperature difference between the theoretical temperature difference and the actual temperature difference is greater than the temperature differential threshold, indicated at 62, then the method further includes indicating a substrate 36 failure, block 64. The failure may be indicated in any suitable manner, including but not limited to displaying a warning light and/or sound, or otherwise scheduling maintenance.

If the absolute value of the temperature difference is less than the temperature differential threshold, indicated at 66, then the method further includes defining a particulate matter threshold, block 68. The particulate matter threshold is the maximum recommended amount of particulate matter deposited or trapped within the particulate filter 34. An amount of particulate matter above the particulate matter threshold may adversely affect the flow of the exhaust gas through the particulate filter 34.

The method further includes detecting a particulate matter level of particulate matter trapped in the particulate filter 34, block 70, and comparing the particulate matter level to the particulate matter threshold to determine if the particulate matter level is greater than the particulate matter threshold, block 72. The particulate matter level is the current level or amount of particulate matter deposited or trapped within the particulate filter 34. The particulate matter level may be determined in any suitable manner, including but not limited to the use of sensors and/or calculations. The particulate matter level may be compared to the particulate matter threshold after comparing the absolute value of the temperature difference to the temperature differential threshold.

If the particulate matter level is less than the particulate matter threshold, indicated at 74, then the method further includes defining a theoretical pressure difference between the upstream end 38 and the downstream end 40 of the particulate filter 34, block 76. The theoretical pressure difference is the pressure difference between the first pressure sensor and the second pressure sensor 48 that should occur if the particulate filter 34 and particularly the substrate 36 are operating properly.

Defining the theoretical pressure difference may include calculating a theoretical upstream pressure and a theoretical downstream pressure of the exhaust gas for the current operating conditions of the exhaust system 20. The theoretical downstream pressure is subtracted from the theoretical upstream pressure to obtain the theoretical pressure difference. For example, an equation may be generated to calculate the theoretical upstream pressure and the theoretical downstream pressure based on the flow rate of the exhaust gas, the temperature of the exhaust gas upstream of the particulate filter 34, or some other known variable of the exhaust system 20.

Alternatively, defining the theoretical pressure difference may include referencing stored pressure values from a table of pressure values for the current operating conditions of the exhaust system 20 to determine the theoretical upstream pressure and the theoretical downstream pressure of the exhaust gas. The theoretical downstream pressure is subtracted from the theoretical upstream pressure to obtain the theoretical pressure difference. The theoretical upstream pressure and the theoretical downstream pressure for specific operating conditions may be determined, for example, through testing at various operating conditions. These values may be stored in memory of the engine control unit, and may be referenced to determine the theoretical upstream pressure and the theoretical downstream pressure of the exhaust gas for the current operating conditions of the engine 22 and the exhaust system 20. Accordingly, the engine control unit may learn the current operating conditions of the engine 22 and the exhaust system 20, and use the current operating conditions to reference the pressure table to determine the theoretical upstream temperature and the theoretical downstream temperature of the exhaust gas.

The method further includes measuring the actual pressure difference between the upstream end 38 and the downstream end 40 of the particulate filter 34, block 78. The first pressure sensor 46 measures an actual upstream pressure of the exhaust gas, and the second pressure sensor 48 measures an actual downstream pressure of the exhaust gas. The actual downstream pressure is subtracted from the actual upstream pressure to obtain the actual pressure difference between the upstream end 38 and the downstream end 40 of the particulate filter 34.

The method further includes calculating an absolute value of the difference between the theoretical pressure difference between the upstream end 38 and the downstream end 40 of the particulate filter 34, and the actual pressure difference between the upstream end 38 and the downstream end 40 of the particulate filter 34, block 80. The absolute value is the real number value of the difference, regardless of a positive or negative sign. Accordingly, the actual pressure difference is subtracted from the theoretical pressure difference to obtain the pressure difference between the theoretical pressure difference and the actual pressure difference. The absolute value is then taken of the pressure difference between the theoretical pressure difference and the actual pressure difference.

The method further includes defining a pressure differential threshold, block 82. The pressure differential threshold is an upper pressure limit associated with proper operation of the particulate filter 34 and/or the substrate 36. Accordingly, a pressure below the pressure differential threshold is indicative of proper functioning of the particulate filter 34 and/or the substrate 36, whereas a pressure above the pressure differential threshold is indicative of a failure of the particulate filter 34 and/or the substrate 36.

The method further includes comparing the absolute value of the difference between the theoretical pressure difference and the actual pressure difference to the pressure differential threshold, block 84. The absolute value of the pressure difference is compared to determine if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is greater than the pressure differential threshold.

If the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is greater than the pressure differential threshold, indicated at 86, then the method may further include indicating a substrate 36 failure, block 64. The failure may be indicated in any suitable manner, including but not limited to displaying a warning light and/or sound, or otherwise scheduling maintenance.

If the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is less than the temperature differential threshold, indicated at 66, and if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is less than the pressure differential threshold, indicated at 88, then the method may further include stopping analysis of the substrate 36 without indicating a substrate 36 failure, block 90.

If the particulate matter level is greater than the particulate matter threshold, indicated at 92, then the method may further include stopping analysis of the substrate 36 without indicating a substrate 36 failure, block 90. The analysis is exited prior to comparing the absolute value of the difference between the theoretical pressure difference and the actual pressure difference to the pressure differential threshold, block 84. If the particulate matter level is greater than the particulate matter threshold, indicated at 92, then the elevated particulate matter level may adversely affect the upstream pressure and/or downstream pressure of the exhaust gas, rendering the analysis between the theoretical pressure difference and the actual pressure difference inaccurate. Specifically, the elevated particulate matter level increases fluid flow resistance through the particulate filter 34, thereby increasing the upstream pressure and/or decreasing the downstream pressure. As such, the pressure analysis to determine if the substrate 36 has failed may be abandoned because the altered actual pressure difference may render the analysis untrustworthy. Therefore, it should be appreciated that the particulate matter level is compared to the particulate matter threshold before comparing the absolute value of the difference between the theoretical pressure difference and the actual pressure difference to the pressure differential threshold.

While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims. 

1. A method of detecting a failure in a substrate of a particulate filter in an exhaust system of a vehicle, the method comprising: calculating an absolute value of a difference between a theoretical temperature difference between an upstream end and a downstream end of the particulate filter and an actual temperature difference between the upstream end and the downstream end of the particulate filter; comparing the absolute value of the difference between the theoretical temperature difference and the actual temperature difference to a temperature differential threshold to determine if the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is greater than the temperature differential threshold; calculating an absolute value of a difference between a theoretical pressure difference between the upstream end and the downstream end of the particulate filter and an actual pressure difference between the upstream end and the downstream end of the particulate filter; comparing the absolute value of the difference between the theoretical pressure difference and the actual pressure difference to a pressure differential threshold to determine if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is greater than the pressure differential threshold; and indicating a substrate failure if the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is greater than the temperature differential threshold or if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is greater than the pressure differential threshold.
 2. A method as set forth in claim 1 further comprising defining a particulate matter threshold.
 3. A method as set forth in claim 2 further comprising detecting a particulate matter level of particulate matter trapped in the particulate filter.
 4. A method as set forth in claim 3 further comprising comparing the particulate matter level to the particulate matter threshold to determine if the particulate matter level is greater than the particulate matter threshold.
 5. A method as set forth in claim 4 wherein comparing the particulate matter level to the particulate matter threshold is further defined as comparing the particulate matter level to the particulate matter threshold after comparing the absolute value of the difference between the theoretical temperature difference and the actual temperature difference to the temperature differential threshold.
 6. A method as set forth in claim 5 wherein comparing the particulate matter level to the particulate matter threshold is further defined as comparing the particulate matter level to the particulate matter threshold before comparing the absolute value of the difference between the theoretical pressure difference and the actual pressure difference to the pressure differential threshold.
 7. A method as set forth in claim 6 further comprising stopping analysis of the substrate without indicating a substrate failure prior to comparing the absolute value of the difference between the theoretical pressure difference and the actual pressure difference to the pressure differential threshold when the particulate matter level is greater than the particulate matter threshold.
 8. A method as set forth in claim 1 further comprising defining the theoretical temperature difference between the upstream end and the downstream end of the particulate filter.
 9. A method as set forth in claim 8 wherein defining the theoretical temperature difference includes calculating a theoretical upstream temperature and a theoretical downstream temperature of the exhaust gas for the current operating conditions of the exhaust system.
 10. A method as set forth in claim 8 wherein defining the theoretical temperature difference includes referencing stored temperature values from a table for the current operating conditions of the exhaust system to determine a theoretical upstream temperature and a theoretical downstream temperature of the exhaust gas.
 11. A method as set forth in claim 1 further comprising defining the theoretical pressure difference between the upstream end and the downstream end of the particulate filter.
 12. A method as set forth in claim 11 wherein defining the theoretical pressure difference includes calculating a theoretical upstream pressure and a theoretical downstream pressure of the exhaust gas for the current operating conditions of the exhaust system.
 13. A method as set forth in claim 11 wherein defining the theoretical pressure difference includes referencing stored pressure values from a table for the current operating conditions of the exhaust system to determine a theoretical upstream pressure and a theoretical downstream pressure of the exhaust gas.
 14. A method as set forth in claim 1 further comprising measuring the actual temperature difference between the upstream end and the downstream end of the particulate filter.
 15. A method as set forth in claim 1 further comprising measuring the actual pressure difference between the upstream end and the downstream end of the particulate filter.
 16. A method as set forth in claim 1 further comprising defining the temperature differential threshold.
 17. A method as set forth in claim 1 further comprising defining the pressure differential threshold.
 18. A method as set forth in claim 1 further comprising stopping analysis of the substrate without indicating a substrate failure if the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is less than the temperature differential threshold and if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is less than the pressure differential threshold.
 19. A method of detecting a failure in a substrate of a particulate filter in an exhaust system of a vehicle, the method comprising: measuring the actual temperature difference between an upstream end and a downstream end of the particulate filter; calculating an absolute value of a difference between a theoretical temperature difference between the upstream end and the downstream end of the particulate filter and the actual temperature difference between the upstream end and the downstream end of the particulate filter; comparing the absolute value of the difference between the theoretical temperature difference and the actual temperature difference to a temperature differential threshold to determine if the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is greater than the temperature differential threshold; comparing a measured particulate matter level of particulate matter trapped in the particulate filter to a particulate matter threshold to determine if the particulate matter level is greater than the particulate matter threshold; measuring the actual pressure difference between the upstream end and the downstream end of the particulate filter; calculating an absolute value of a difference between a theoretical pressure difference between the upstream end and the downstream end of the particulate filter and an actual pressure difference between the upstream end and the downstream end of the particulate filter; comparing the absolute value of the difference between the theoretical pressure difference and the actual pressure difference to a pressure differential threshold to determine if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is greater than the pressure differential threshold when the particulate matter level is less than the particulate matter threshold; and indicating a substrate failure if the absolute value of the difference between the theoretical temperature difference and the actual temperature difference is greater than the temperature differential threshold, or if the absolute value of the difference between the theoretical pressure difference and the actual pressure difference is greater than the pressure differential threshold.
 20. A method as set forth in claim 19 wherein comparing a particulate matter level of particulate matter trapped in the particulate filter to a particulate matter threshold is further defined as comparing the particulate matter level to the particulate matter threshold after comparing the absolute value of the difference between the theoretical temperature difference and the actual temperature difference to the temperature differential threshold. 